This invention relates to electrostatography. More particularly, this invention relates to corona generating devices for applying electrostatic charge onto a suitable surface.
In the electrostatographic process, an electrostatographic plate comprising a photoconductive insulating material on a conductive backing is given a uniform electric charge over its surface and then is exposed to the subject matter to be reproduced usually by conventional projection techniques. This exposure discharges the plate areas in accordance with the radiation intensity which reaches them and thereby creates an electrostatic latent image on or in the plate coating which may then be developed into visible form by applying a developer material, e.g., a powder, to the plate using any one of a number of development techniques generally known and used in the art. The developer material electrostatically clings to the plate in a visual pattern corresponding to the electrostatic image. Thereafter the developed image is usually transferred from the plate to a support material such as paper to which it may be fixed by any suitable means thereby forming a permanent print.
Instead of being developed by means of a powder, the electrostatic latent image may also be developed by using liquid development techniques also well known in the art.
The charging of the electrostatographic plate in preparation for the exposure step can be accomplished by means of a corona generating device whereby electrostatic charge is applied to the electrostatographic plate to raise it to a potential of approximately 500 to 600 volts. One form of a corona generating device for this purpose is disclosed in U.S. Pat. No. 2,777,957 wherein a plurality of parallel wires are connected in series to a high voltage source and are supported in a conductive shield that is arranged in closely spaced relation to the surface to be charged. When the wires are energized, corona is generated along the surface of the wires and ions are caused to be deposited on the adjacent photoconductive surface. Suitable means are usually provided to effect relative movement of the surface to be charged and the corona generating device.
It is important that the coronode in the above arrangements be maintained in a condition in which each point along its surface is equidistant from the surface to be charged since variations in this distance will result in non-uniformity of the charge applied to the surface, usually the electrostatographic plate. In order to ensure that the coronode is maintained in horizontal plane equispaced from the imaging surface, it has heretofore been proposed to connect the coronode wires to at least one of the end mountings by means of a tension spring. This has the desirable feature of enabling the wires to be mounted in the device under tension while reducing the possibility of the wires, which are extremely fine (generally of the order of 0.004 inch thick), being stretched beyond their elastic limit during asembly.
It is also desirable in corona generating devices to provide an arrangement for easily replacing a deteriorated coronode or corona electrode with a new one. Since this replacement usually takes place at a commercial site of a machine by a service technician, ease of replacement and adjustment in a minimum amount of time is essential.
In addition, as disclosed in copending application Ser. No. 595,656, filed in the joint names of T. Davis and G. Safford, it has been noted that outstanding charging characteristics are obtainable for xerographic charging by the use of a coronode in the form of a thin wire coated with thick dielectric coating. In such an arrangement, a corona generating A.C. voltage is applied to the wire and a D.C. bias voltage is applied to an adjacent conductive plate to produce a charge control field. Since the details of the coronode and the arrangement for applying corona generating and control voltages thereto form no part of this invention, incorporation by reference of the details disclosed in that application is hereby made.
In addition, as contracted with prior art coronodes in which tension has been applied to a wire member it has been found desirable in coronodes of the type disclosed in application Ser. No. 595,656 to apply the tension to the outer dielectric coating rather than to the inner wire conductor. This latter type of arrangement has the advantage that a wire of less tensile strength may be employed, thereby reducing the cost of the coronode.
In addition, it has been found that the cobmined action of a continuous support for the coronode and a concurrent tensioning of the dielectric coating gives improved performance while facilitating assembly of the device.
When using coronodes of the type disclosed in Ser. No. 595,656 the problems noted above experienced with conventional coronodes become even more acute. Specifically, there is a need to put a tension on the coronode to hold it in a preselected position with respect to the surface to be charged. But since the coronode is comprised of a thin outer coating of a brittle dielectric material, such as glass, it is easily fractured. Thus, in handling the coronode for insertion into the end supports of the corona device it is easy to crack or split the outer sleeve which results in a consequent irregularity in the charge delivered by the device to an imaging surface.